JJ(ジェイ ジェイ,Jei Jei?) is an action game developed and published by Square for the Famicom in 1987. JJ is short for Jump'in Jack, and the game also bears the subtitle of Tobidase Daisakusen Part II, being the sequel to Tobidase Daisakusen (released in North America as 3-D WorldRunner). Unlike that game, JJ was only released in Japan. JJ was the last game by Square to utilize 3D glasses, and was Square's last work before Final Fantasy was released; JJ was released a mere week and a half before.

Like the original game, JJ is a strange cross between Space Harrier and a platformer. Once again, the main character Jack must travel through various worlds, armed with his laser cannon and jumping ability, to defeat the serpentbeasts who have overrun the planets. JJ takes the Super Mario Bros. 2 approach to sequel design; take the same base game and make it tougher and more interesting for experts of the original. However, JJ also sports a surprisingly dark aesthetic and a (mostly) new soundtrack of eight songs, serving the same purposes as the eight songs of the original Tobidase Daisakusen. The sequel was also scored by Nobuo Uematsu, and is Uematsu's 16th work of video game music composition.

Although the game was only released in Japan, it is entirely playable by non-Japanese speaking players. Regardless, the group DvD Translations released a patch which attempts to translate the title, and translates the ending text to English. The patch can be found here.

Although the two games were released only months apart, JJ showcased many graphical improvements over its predecessor 3-D WorldRunner. The sprites have been overhauled, while more attention was paid to small details; when fighting bosses in JJ, for example, Jack is shown with a jetpack. The world backdrops are also richer in detail.

Like 3-D WorldRunner, JJ supported a "3-D mode" where selecting the 3-D mode and wearing 3D glasses makes the game appear in three dimensions. The glasses used are different, however. It has been noted that whereas the 3-D mode in 3-D WorldRunner only required red/blue 3D glasses, the 3-D mode in this game requires special 3D glasses whose left and right lenses switch on and off for every scan of the TV to give a perfect 3D effect. These were the special Famicom 3D System glasses, released only in Japan. These glasses were similar in their operation to the Segascope glasses released for the Sega Master System.